I know Ben has been waiting for this all day, so here goes.
As he mentioned, I plugged it in early this morning as soon as the sun started hitting it. Something to note, I keep my house at 60F during the night and 69 during the day. So, some of the initial heating probably came from the house as well as the panel. But, to keep this all pretty short, I made up a nice graph.
Anyway, the panel didn't heat up the tank nearly as much as I had hoped. But, I did get a good look at it when I did the measurements around noon time. Having it in the window really limits the amount of sunlight hitting the panel. I'd estimate at best, only 1/2 to 2/3rds of the panel is being hit by direct sunlight. This is a severe limitation. Also, I noticed that the mirror hangers are not sealing the glass to the case, so I'm sure there is a fair amount of heat loss there.
As Ben mentioned, it was quite sunny today, as good as can be expected for this time of year IMO. Once we get one of these in the open getting full sun and sealing up the glass I'm sure we'll see much better performance.
Raw data:
